Re: "SELinux: ebitmap: truncated map" after editing with libsepol

[Stephen Smalley <sds@tycho.nsa.gov>]

On 12/17/2014 04:30 AM, Andrew Gunnerson wrote:
> Hello all,
> 
> I have a very simple test program to help with debugging my Android
> dual booting project. It reads the current policy from
> /sys/fs/selinux/policy,
> changes a single type to be permissive, and then loads the new policy
> by writing it to /sys/fs/selinux/load. The problem is, after editing the
> policy with sepol, it fails to load and the kernel prints the following
> message in dmesg: "SELinux: ebitmap: truncated map".
> 
> The program reads and writes the policy file using the standard fopen
> and policydb_read/policydb_write calls. I then set a few types to be
> permissive using the following loop:
> 
>     ...
>     char *name;
>     int is_permissive;
>     char **types = (null terminated char* array)
>     char **type;
>     ...
>     for (unsigned int i = 0; i < pdb->p_types.nprim - 1; i++) {
>         name = pdb->p_type_val_to_name[i];
>         is_permissive = ebitmap_get_bit(&pdb->permissive_map, i + 1);
> 
>         if (!is_permissive) {
>             for (type = types; *type; type++) {
>                 if (strcmp(*type, name) == 0) {
>                     ebitmap_set_bit(&pdb->permissive_map, i + 1, 1);
>                     break;
>                 }
>             }
>         }
>     }
>     ...
> 
> I've been trying to debug this for many hours, but I can't seem to figure
> out why this is happening. Is there a simple mistake I'm overlooking or
> am I approaching this in a completely wrong way?
> 
> Thanks in advance! Any help is greatly appreciated!
> 
> Andrew Gunnerson
> 
> 
> PS: This is running on Android 5.0 with libsepol 2.4-rc4 and kernel
> 3.4.0-g88fbc66.

The implementation of /sys/fs/selinux/load requires you to write the
entire policy in a single write(2) call, so you can't use stdio methods
for writing the policy image.  policydb_write() the image to memory and
then call security_load_policy() on that memory region.

Also, you can see a working example of a program that does this kind of
thing (but on files rather than directly to /sys/fs/selinux/load) in
sepolicy-inject,
https://bitbucket.org/joshua_brindle/sepolicy-inject

Any particular reason you are building a pre-release upstream libsepol
rather than the one included in AOSP (external/libsepol)?  Admittedly,
that is only built for the host, not the device, presently, so you'd at
least need to change that.